iwlwifi: update script to extract firmware/chipset generations/flavors/..

Adjust the logic for using the sysctl vs. providing an input file
(previously generated from the sysctl).

Adjust template generation for iwlwififw.4 to also list the flavour
for each chipset (if known) to help people manually building packages
to figure out which port flavor they need.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days